Process for the heat-treatment of metals



Patented Sept. 4, 1945 PROCESS FOR THE HEAT-TREATMENT OF METALS Alfred Gordon Evans Roblette and Peter Francis Hancock, Erdington, England I No Drawing. Application July 8, 1942, Serial No.

450,204. In Great Britain June 11, 1941 Q'Clalms. (01448-2111) This invention has reference to a process of heat treating iron-or alloy iron castings.

The principal object of the invention is to I render brittle iron or alloy iron castings, tough,

ductile and, if desired, wear resistant.

Another object of the invention is to subject iron or alloy iron which has a high carbon content, and is capable of being cast easily. to a heat treatment whereby the carbon content is reduced to a value comparable to the carbon content of steel, the melting and casting of which calls for a special highly-skilled technique.

A further object of the invention is to impart to iron or alloy iron castings such physical prop- :rtgels that the metal becomes comparable with A further object of the invention is to enable articles, which heretofore could only be produced from steel, to be produced from iron or alloy iron castings.

A still further object of the invention is to devise a heat treatment process which requires a minimum of skilled labour and which enables tough, ductile and, if desired, wear resistant articles to be produced in a short period of time from iron or alloy iron castings, in a simple and efllcient manner.

hydrocarbon-containing gas such as coal gas, coke oven gas and theili ke.

The subsequent carburising treatment prefer- I ably consists in heating the decarburised castings in a furnace in a gaseous atmosphere of controlled composition or, alternatively, a solid or liquid carburising medium may be used.

To obtain a more uniform carbon content throughout the treated castings, the latter may be further heated in 81188830115 atmosphere of a composition which is so controlled that the gas is neutral to the desired carbon content, whereby the said carbon content is caused to diffuse Further objects of the invention will be apparent from the following description and claims.

The invention consists, primarily, in subjecting iron or alloy iron castings to a decarburising heat treatment whereby carbon is removed progressively from the surface zones so that a carbon content gradient is produced from the surface to the core, the said castings being then sub-. jected to a carburising heat treatment whereby a substantially uniform and predetermined average carbon content is provided throughout the resulting metal.

final carbon content may vary' slightly from the surface to the core and at the inter-. vening zones, and the resulting metal may, if desired, be subjected to a further treatment on treatments for providing or developing a hard,- wear-resistant skin or zone on the ouside of the castings.

, Thedecarburising treatment consists in heating the castings in a furnace in a gaseous atmosphere containing some or all of the constituents hydrogen, water vapour, carbon dioxide, carbon monoxide and nitrogen. Such an atmospliere may be produced by the controlled combustion of solid carbonaceous material, or by the controlled combustion of producer gas or a throughout the castings.

Thus the carbon content of thecastings may be varied in any desired manner, to provide tough and ductile metal which, if desired, may be surface hardened in any known manner. Alternatively, the product of the heat treatment may be subjected to additional treatment which may involve quenching an'd/or tempering, the treatment being chosen according to the physical properties required and to the composition of the initial castings. I

A typical process, in accordance with the invention, consists in decarburising white iron castings, having an average section three-eighths of an inch in thickness and,a composition of 2.0 to 4.0% carbon, 0.3 to 2% silicon, 0.4 to 2% manganese, up to 0.15%. phosphorus and up to 0.2% sulphur, with or without additions of other alloying elements, for a period of fity hours in an atmosphere produced by the controlled combust on of coal gas, the treatment temperature be in the region of 1050 C. I

The product of this decarburising treatment consists of a core of pearlite, a surface or skin of ferrite which is practically free of carbon, and a transition zone in which the structure changes gradually fromthe ferritic skin to the pearlitecore and in which the carbon content increases gradually towards the core.

The steepness of the carbon content gradient from the skin to the core and the rate of removal of the carbon, are influenced by the duration and temperature of the decarburising treatment and by the composition of the decarburising atmosphere. Hence any desired carbon con the metal, namely in the region of 1000 to 1100 C. Further the gaseous atmosphere is as decarburising as possible and contains one or more of the constituents hydrogen, carbon dioxide and a certain amount of water vapour, as decarburising agents, together with carbon monoxide as a reducing agent to prevent the oxidation of the iron and, to some extent, of any alloying' metal such as manganese; the hydrogen also serves as a reducing agent. Generally'speaking the atmosphere will also contains proportion of nitrogen which is inert.

The decarburising step is terminated when the desired carbon content is obtained at the core and the material is then subjected to the next step of carburisation.

In the above mentioned typical process, the \carburising treatment consists in heating the material for a period of ten hours, at a temperature of 1000 C., in an atmosphere of carburising gas which may be a hydrocarbon-containing gas such as neutral gas. propane or butane mixed-or diluted, if desired, with a proportion of substantial- -ly neutral or inert constituent. Alternatively a solid or liquid carburising medium may be used.

The product of the carburising treatment is substantially pearlite throughout, except that there may exist at the surface a slightly higher carbon content than at the core. However, if desired, the treatment may be so controlled that the carbon content at the surface is equal to, or ,slightly less than, the content at the core since' the reintroduction of carbon into the castings is determined by the duration and temperature of' recarburisation and to some extent by the properties of the carburising medium. The temperature and composition of the medium control the carbon content on the surface and the duration of treatment affects the depth to which 'carburisation proceeds.

Finally, if desired, when the recarburising treatment has proceeded to the desired stage, the supply of carburising gas is cut of! from the furhate the castings are subjected to a further pe-- riod of heat treatment in an atmosphere of the neutral or inert constituent of the carburising atmosphere, so as to permit of the diil'usio'n of the carbon content, andallowed to cool in the said constituent. The treated castings are then removed from the furnace, and may be given any desired ;i lnishing treatment such as surface hardening.

Alternatively, the usual slightly higher carbon content at the surface may be retained by subjecting the recarburised castings to the hardening treatment immediately after the said carburisation step is complete.-

would occur with a carbon content of the order of 0.9% to 1.8%; the surface or outer zone,,having a lower carbon content than the core, containing more or less martensite. Such a structure may then be made substantially austenitic throughout by the recarburising treatment. For

certain Purposes it may not be necessary to remove all the original carbide from the core so reduce the carbon content at the core. and thereafter heating the 'deca'rburized product having said carbon content gradient in a carburizing medium until substantially the same carbon content is obtained at the surface as at the core of said decarburized product, whereby a steel-like prod- Y uct is obtained having a lower carbon content than the original cast iron treated.

2. Aprocess for treating white cast iron containing over 2% carbon-which comprises decarburizing said cast iron at a temperature of at least about 1000 C. but below the melting point in a gaseous medium which is non-oxidizing to iron for a period of time suiilcient to produce a carboncontent gradient which increases toward the core and to decrease the carbon content of the core, and thereafter carburizlng the decarburized product having said carbon content gradient at a temperature of at least about 1000 C. in a gaseous carburizing atmosphere for a period of time sufficient toproduce substantially the same carbon content at the surface as in the vicinity of the core, said carbon content being substantially the same as that of steel.

3. A process for treating white cast iron to obtain a product having properties analogous to steel which comprises decarburizing the mass of said cast iron at a temperature below the meltins int but.not less than aboutl000 C. in a gaseous medium which is non-oxidizing to iron for a period of timesufllcient to produce a car the surfaceas in the vicinity of the core and Alternatively the recarburised castings may be 7 quenched from carburising temperature in air,

oil or water, and subsequently tempered to give a hard tough structure throughout themetal.

The physical properties imparted to the castings by the above-described process are comparable to those of steel of corresponding analysis with the result that articles which, heretofore, have red to be produced from. steel castings may be produced from iron castings, thereby not only reducing production costs, but also avoiding the necessity of employing highly skilled labour.

' The process is. particularly applicable to castings of alloy irons such as austenitic irons containing manganese. For example, a white iron .containing any carbon content and from 10% to 19% manganese, may be decarburised so that the I core contains substantially no tree carbide. This characterized by Properties similar to those of steel of corresponding analysis.

4. A process for producing steel-like produbts from cast iron containing over 2% carbon which comprises decarburizing said cast iron at a temperature of at least about 1000C. but below the melting point in a gaseous non oxidizing medium until a carbon content gradient is produced which increases toward the core and until the carbon content of the core is decreased, and thereafter heating. the decarburized product having said carbon content gradient in a gaseous carburizlng medium until substantially the same carbon content is obtained at the surface as in the vicinity of the core, the carbon contentat the surface being at least as great as the carboneontent ail- Jacent the core, whereby a product is obtained characterized by properties similar to those of steel of corresponding analysis. I

and thereafter carburizing the decarburized product having said carbon content gradient for a period of time sufiicient to-produce a substantially austenitic structure throughout the mass of the product. 6. A process for treating white cast iron con-' taining about 10% to 19% of manganese and having a micro-structure containing austenite and carbide which comprises decarburizing said white cast iron at a temperature below the melting point for a period of time suflicient to produce a carbon content gradient which increases toward the core and to remove at least some of the carbide in the core; and thereafter carburizing the decarburized product having said carbon content gradient for a period of time sufiicient to produce a zone near the surface in which the structure is substantially all austenitic, whereby a product is obtained having a core in which the structure is an austenitic matrix having carbide impedded therein and having a zone near the surface in which the structure is substantially all austenitic.

7. A process for treating manganese white cast iron having a microstructure containing austenite and carbide which comprises decarburizing said white cast iron at a temperature of at least about 1000 C. but below the melting-point in an atmosphere which is non-oxidizing to iron for a period of time suilicient to produce a carbon content gradient which increases toward the core and to reduce the carbon content near the surface to below about 0.9% carbon, and thereafter carburizing the decarburized product having said carbon content gradient to obtain a carbon content of at least about 0.9% substantially throughout the mass of the product.

8. A process for treating manganese white cast iron having a microstructure containing austenite and carbide which comprises decarburizing said white cast iron below the melting point for a time sufficient to produce a carbon content gradient which decreases from the core toward the surface and to reduce the carbon content near the surface-from that of the carbide-containing cast iron to a carbide-free material containing substantially less than 0.9% carbon, and thereafter carburizing the decarburized product having said carbon content gradient to increase the carbon content and to obtain a product having at least 0.9% carbon substantially throughout the mass of the product.

9. A process for producing a steel-like product from white cast iron which comprises decarburlzing said white cast iron at a temperature below the melting point for a period of time sufficient to produce a carbon content gradient which increases from the surface toward the core of said cast iron and to reduce the carbon content at the core, and thereafter heating the decarburized product having said carbon content gradient in a carburizing medium until substantially the same carbon content is obtained near the surface as near the core of said decarburized product, whereby a steel-like product-is obtained having a lower a carbon content than the original cast iron treated.

ALFRED GORDON EVANS ROBIETTE. PETER FRANCIS HANCOCK. 

